1. Field of the Invention
This invention pertains to cylindrical threaded connections and particularly to such connections employed on tubular goods used in the oil and gas industry.
2. Description of the Prior Art
Tubular goods generally comprise a plurality of individual joints each having a pin end and a box end. A connection comprises the pin end of one joint made up with the box end of the adjoining joint. Such ends can either be tapered or substantially cylindrical in configuration. For those connections that are cylindrical, the cylindrical ends are considered to have a nominal radial dimension that remains constant from one revolution to the next at corresponding parts of the thread (e.g., root dimension, crest dimension).
Cylindrical ends are rarely ever truly cylindrical. Often threads are made by milling the external threads on the pin end and the internal threads on the box end. Certain milling inherently produces a slight taper to both the pin end and the box end such that the smallest radial dimension of the pin end is in the first thread revolution and the largest radial dimension of the box end is in the first thread revolution.
It is also common for the threads on both the pin end and the box end to include two thread sets separated by a step so that the thread set on the pin end nearest its tip is at a reduced radial dimension with respect to the other and larger dimension thread set. Correspondingly, the thread set of the box end closest to the open end is at a larger radial dimension than the second thread set located deeper within the opening of the box end.
Cylindrical threaded connections are conventionally made to be free running through the majority of revolutions of their engagement. That is, there is radial clearance between the crest of the threads on the pin end and the root of the threads on the box end. Hence, there is no friction of the crest-and-root of the threads as the first several revolutions are made. Because of the slight taper previously mentioned introduced by the milling process, as the threads approach near complete makeup the radial clearance could reduce to the point of interference for the last few threads only so that more and more torque has to be applied until complete makeup is achieved.
The metal-to-metal seal or shoulder contact is achieved in the cylindrical connection just described in one or more of three surfaces: in the metal area toward the tip from the lead thread on the pin end, in the step area between the thread sets, and in the area behind the last thread on the pin end. The step area between the thread sets is a torque stop shoulder and does not provide a seal, as do the other locations. Engagement is generally only on the initial assembly. Because of permanent distortion caused by the initial assembly of parts, on subsequent assemblies the effect of contact in this region is not great. In the prior art connection just described, there is a danger of galling one or more of these surfaces as the connection is tightened in the final revolution or so of makeup as the sharp line of merging tapered surfaces are forced together.
It may also be apparent that the milling of the threads does not affect the flat seal areas; therefore, the milling taper introduced at the threads may not be introduced at the seal areas, which ideally are maintained at a similar degree of taper.
U.S. Pat. No. 4,244,607 describes a connection in which the pin end intentionally is provided with a small amount of taper with respect to the box end member. That is, there is more taper to the pin end than to the box end. Therefore, the threads are free running until the last few threads engage. The torque becomes greater and greater until makeup is completed. Such a structure merely exaggerates the standard product, possibly also introducing some contact at the load flank of the thread in addition to gradually eliminating the free-running radial clearance. Galling is not minimized with respect to the standard thread design. The loading of threads is not evenly distributed, but is primarily on the threads making the heaviest contact (i.e., having the greatest breakband effect).
Therefore, it is a feature of the present invention to provide an improved cylindrical threaded connection which is free running except for the introduction of interference in the area of the last revolution or two before complete connection makeup without the shortcomings present in the connection described in U.S. Pat. No. 4,244,607.
It is still another feature of the present invention to provide an improved cylindrical threaded connection having a more controlled interference, heavy torque area of contact than connections in the prior art.
It is another feature of the present invention to provide an improved cylindrical threaded connection which provides an evenly distributed torqued area than connections in the prior art.
It is still another feature of the present invention to provide an improved cylindrical threaded connection which provides an enlarged amount of even distributed torqued area which permits the advantages of an evenly made connection even if there are some scarring in part of such area.
It should be further noted that the present invention provides the further advantage of permitting joints invention-enhanced with connector ends to be interchanged or intermingled with joints without the features described herein. For connections having a pin end or box end having threads enhanced in accordance with the present invention, the connection is improved in accordance herewith, even when the mating end does not include the enhancement.